Site-Specific Doping of Mn2+ in a CdS-Based Molecular Cluster

The synthesis and characterization of a CdS-based molecular cluster, [Cd10S4(SPh)16]4– (Cd10), with site-specific substitution of Cd2+ with Mn2+ impurities are reported. The formation of the Cd10 cluster from the smaller [Cd4(SPh)10]2– (Cd4) cluster involves a metastable intermediate cluster, [Cd8S­(SPh)16]2– (Cd8), that is detected by electrospray ionization mass spectrometry (ESI-MS). To account for this unexpected intermediate, we propose a complex equilibrium between Cd4, Cd8, and Cd10 exists that we exploit to introduce Mn2+ impurities at both core and surface cation sites of the Cd10 lattice. We demonstrate through two synthetic procedures that differ only in the sequence in which Mn2+ is introduced to the reaction dictates its speciation in the cluster. Introducing dopants at an early stage of the synthesis prevents full conversion of Cd8 to Cd10; however, it yields core doped Cd10 clusters. Addition of Mn2+ ions after the preparation of Cd10 yields only surface doped clusters. The composition of the doped clusters is systematically characterized by ESI-MS and exhibits speciation-dependent peak intensities. Photoluminescence (PL) spectra of the Mn2+-centered 4T1 → 6A1 transition also exhibits significant differences in peak position and PL lifetimes that are consistent with the expected variation in ligand field strength experienced by these two metal sites. However, ESI-MS and PL collected on “aged” samples indicate slow displacement of Mn2+ from core sites. This study provides new insights to the growth mechanism of clusters that remained rather elusive and demonstrates how the cluster surface dynamics and cluster equilibria can be exploited for precise doping of these well-defined semiconductor analogues.